Techniques used to study human gender differences in cognition are limited by the problem of separating biological substrates from environmental and social factors. The relative weightings of the latter factors would be reduced if cognitive gender differences in humans were demonstrated by using tests that were originally used with monkeys to show gender differences. Furthermore, the argument in favor of biological factors would be especially strong if the direction of the gender differences in monkeys on these tests could be reversed by hormonal manipulation. Two such tests are object reversal and concurrent object discriminations. Experiments in infant monkeys have shown that males learn the former task significantly earlier in life than do females. In contrast, infant female monkeys learn the latter task significantly earlier than do males. On both tasks, the direction of gender differences can be reversed by manipulation of perinatal hormones and/or neonatal ablations of specific brain sites. Since the two tasks are similar except in their cognitive demands, this double dissociation in infant monkeys provides strong evidence that some cognitive gender differences are due to differential maturational rates of specific brain areas which are under the influence of hormones. In order to determine whether this behavior pattern may also exist in humans, the proposed research will test normal children and adults with these two behavioral procedures. Children, ranging in age from 12 to 36 months, and adult controls will be tested longitudinally on the two tasks, using testing procedures which have been designed to resemble as closely as possible those used with monkeys. These general procedures have been effective previously with children. Pilot data indicate that young male children do, in fact, master a series of object reversals earlier than do females, and, in contrast, young females appear to master concurrent discriminations earlier than males. Because all gender differences involve some degree of overlap between the sexes, many more subjects are needed before statistically significant conclusions can be reached.